Measurements of liquid base film thickness distribution have been obtained for 206 horizontal annular two-phase (air-water) flow conditions in 8.8 mm, 15.1 mm, and 26.3 mm ID tubes. It is found that the trends in base film thickness measurement do not match trends in the literature for average film thickness, which considers waves and base film together. An iterative critical friction factor model is used to model circumferentially-averaged base film thickness; an explicit, empirical correlation is also provided. Asymmetry is well-correlated by a modified Froude number based on the correlated base film thickness and the gas mass flux. The iterative model is also extended to estimate the critical film flow rate. (C) 2009 Elsevier Ltd. All rights reserved.